86 
Journal of the 
v.’hich the plane of vibration was in accord with the edge, o — it, 
but formed low angles up to 11° * nor were any of these 
crystals rectangular in outline. Some showed zonal growth, in 
which the zones were marked by minute inclusions of ground-mass, 
or of devitrified basis. Inclusions in these felspars are rare, and 
are principally confined to apatite. Flakes of greenish chlorite 
are numerous, but these are rather alteration products than 
inclusions. 
In slices from another dyke I observed sections of felspai's in 
which the angle formed by the plane of vibration with the com¬ 
position face in the zone, o—ii were 11°, 14°, 17°, 27°. The 
crystals in this case were compounded according to the Albite and 
Carlsbad law, and also grouped irregularly together. Almost the 
only inclusions in these felspars are very small, long and colour¬ 
less prisms, probably apatite. They are, in some cases, arranged 
according to the planes of growth in the crystal—(Fig. 5.) I have 
also observed a few instances of minute colourless microliths in 
the felspars, having an apparently monoclinic habit, and finally also 
a few rare cases of inclusions of magnetite. 
Many of the triclinic felspars have undergone alteration. This, 
it seems, commences by the production of a grey cloudy substance, 
either in the centre of the crystal, or extending into it from 
without, and connected with similar grey cloudy patches in the 
ground-mass. The ultimate result of this process is that the 
felspar, although still polarising as a doubly refracting substance, 
quite loses the bright chromatic effects which characterise its fresh 
state; this I believe to be kaolinization. In almost in all cases, 
however, I have observed some few shreds of the original felspar 
substance remaining, in which the banded structure is still perfect; 
these portions are frequently round the exterior of the crystal. 
Another frequent change is caused by the presence of a dark, 
blackish green granular or flaky substance, which is, I do not 
doubt, a chlorite, and which, in some cases, surrounds the margin 
while in other cases, it follows the line of cleavage and fissures* 
(Figs. 5 and 6.) 
Another process of alteration consists in the production of 
minute scales of some mica which, in many cases, almost replace 
the whole of the original crystal. This alteration of felspar into 
mica is well known, and as the felspars in this case are of the 
